OnWindowActivate property (ExcelListener).

n Excel instance may contain several workbooks. Only one of these workbooks
may be visible. The "window activation." event is fired when another workbook
comes into focus. The OnWindowActivate property provides facilities for
trapping of such event. The code below provides an example. The output of the
signature() method indicates that the event handler must take two arguments.
We provide the event handler and pass it to the "bind" method.

Consequently, if the user opens another workbook then the ControlShell
receives output.